Cellulose diacetate is the acetate ester of cellulose and is used for a variety of products, including textiles (e.g., linings, blouses, dresses, wedding and party attire, home furnishings, draperies, upholstery and slip covers), industrial uses (e.g., cigarette and other filters for tobacco products, and ink reservoirs for fiber tip pens, decking lumber), high absorbency products (e.g., diapers, sanitary napkins, and surgical products), thermoplastic products (e.g., photography film, plastic instruments, and tape), cosmetic and pharmaceutical (extended capsule/tablet release agents and encapsulating agent), medicinal (hypoallergenic surgical products) and others.
Cellulose diacetate typically has a degree of substitution (DS) of approximately 2.5 and is made commercially by a two-step reaction: (1) esterification and (2) hydrolysis. Esterification involves acetylating cellulose with acetic anhydride, acetic acid as a diluent and sulfuric acid as a catalyst. These reactants are mixed with cellulose until the DS reaches to about 3. Due to the exothermic nature of this reaction, temperature control is vital to ensure the final product parameters are met. After esterification, the catalyst concentration is neutralized and water is added to initial the hydrolysis reaction. The DS of the polymer is reduced from 3 to approximately 2.5.
Typically 2 to 15 wt. % of sulfuric acid is necessary per weight of starting cellulose material. The use of sulfuric acid in this process results in the production of derivatives having chemically bound acid sulfate group. Because these groups have a degrading effect on the cellulose acetate at elevated temperature, they must be inactivated by treatment with hard water or removed by steam. Also, by the end of the reaction, the sulfuric acid catalyst is neutralized by a metal salt and discarded. In addition, sulfuric acid is highly corrosive, difficult to handle, and can damage equipment.
The need exists for improved processes for producing cellulose acetate from raw materials, and in particular to processes for forming cellulose acetate that do not involve the use of sulfuric acid.